1. Field of the Invention
This invention relates to an analog to digital converter which employs plural .DELTA..SIGMA. modulators so as to be advantageously employed for e.g. a digital audio tape recorder. More particularly, it relates to an analog to digital converter which, while employing plural .DELTA..SIGMA. modulators, is able to simplify the circuit construction as well as to improve the S/N ratio.
2. Related Art
There is currently known a digital audio tape recorder (DAT) for converting analog audio signals supplied from external equipment, such as a CD player, into digital data and recording or reproducing the audio data on or from a magnetic tape.
The DAT is known to suffer from deterioration in sound quality and recording/playback noise only to a limited extent because it records and reproduces analog audio signals after translation into digital audio data.
A conventional analog to digital converter (A/D converter) comprises a first .DELTA..SIGMA. modulator 101 and a second .DELTA..SIGMA. modulator 103, supplied with analog audio signals in parallel, a first decimation filter 102 connected in series with the first .DELTA..SIGMA. modulator 101, a second decimation filter 104 connected in series with the second .DELTA..SIGMA. modulator 103, an additive node 105 for summing output data from the first .DELTA..SIGMA. modulator 101 and output data from the second .DELTA..SIGMA. modulator 103, an attenuator 106 for attenuating audio data from the additive node by one half and a quantizer 107 for re-quantizing audio data from attenuator 106, as shown in FIG. 1, to improve the S/N ratio.
The audio signals are supplied via an input terminal 100 to the first .DELTA..SIGMA. modulator 101 and the second .DELTA..SIGMA. modulator 103 of the A/D converter.
The above-mentioned first .DELTA..SIGMA. modulator 101 samples and quantizes the audio signals with sampling clocks of e.g. 3.072 MHz or 48 kHz.times.64 to translate the audio signals into 1-bit audio data which is supplied to the first decimation filter 102.
Similarly, the second .DELTA..SIGMA. modulator 103 generates 1-bit audio data which is supplied to the second decimation filter 104.
Meanwhile, the .DELTA..SIGMA. modulators 101 and 103 translate the quantization errors generated on quantization into analog data which is subtracted from audio signals supplied via input terminal 100 for eliminating quantization errors.
The first decimation filter 102 eliminates, from the 1-bit audio data, the high-frequency noise generated on .DELTA..SIGMA. modulation and the high-frequency noise mixed into the audio signals, while translating the 1-bit audio data into 16-bit 48 kHz audio data which is supplied to the additive node 105. Similarly, the second decimation filter 104 translates the 1-bit audio data into 16-bit 48 kHz audio data which is supplied to the additive node 105.
The additive node 105 sums the 16-bit 48 kHz audio data supplied from the first decimation filter 102 and 16-bit 48 kHz audio data supplied from the second decimation filter 104 to supply the summed audio data to attenuator 106.
By the operation of summing e.g. M-bit audio data from the first and second decimation filters 102, 104 by the additive node 105, a 1-bit carry is produced at the MSB so that output audio data from additive node 105 is (M+1) bit audio data. For this reason, the audio data from the additive node 105 is supplied to attenuator 106.
The attenuator 106 executes a division by multiplying the audio data by 1/2. The resulting quotient data is supplied to quantizer 107.
The quantizer 107 re-quantizes the audio data from attenuator 106 and restores the (M+1) bit data from node 105 to the original M-bit data which is output-ted at an output terminal 108.
Since the above-described A/D converter sums audio data from the first and second .DELTA..SIGMA. modulators 101, 103 at the additive node 105, the S/N ratio of the output audio data from output terminal 108 may be improved as a principle by 3 dB.
Although with the above-described conventional A/D converter it is possible as a principle to improve the S/N ratio of the audio data by 3 dB, since in-phase noise components are also summed when the two audio data are summed together at the additive node 105, the in-phase noise components are doubled in signal level.
Besides, since the carry produced by the summing operation is compensated by re-quantization by the quantizer 107, the re-quantization noise produced, with the result that the S/N ratio was deteriorated, should be improved by 3 dB.
The result is that the summing operation by the two .DELTA..SIGMA. modulators 101, 103, is only of small import.
In addition, since it is necessary to provide a decimation filter for each of the .DELTA..SIGMA. modulators 101, 103, the number of component parts is increased with consequently increased circuit scale and costs.